Plasticity, or neuroplasticity, is the ability of the brain to reorganize its neural circuits. Thanks to this skill, our brain can learn and develop new things from infancy to adulthood and recover after brain damage due to stroke or trauma.
Our brain has the capacity to process a lot of information at the same time. Although different parts of the brain are specialized for different tasks, they control each other and can compensate for minor errors. In brain damage due to stroke or trauma, significant tissue losses occur in the brain and the lost functions may not be compensated immediately. However, thanks to plasticity, the brain can reprogram itself and show an improvement over time.
As in child development, the basis for establishing new neural connections is to be in an environment rich in sensory and movement stimuli. The more stimulation patients with stroke and brain injury receive in terms of sensation and movement, the better the chance and degree of recovery will be. Examples of such stimuli are exercises, robotic rehabilitation, virtual reality, mental exercises, transcranial stimulation, music therapy.
The basic structure of the brain is determined by genes while still in the womb. However, the brain continues to develop after birth. Acquiring new skills as a child grows is associated with a process called developmental plasticity. In developmental plasticity, nerve cells (neurons) and their connections (synapses) change. Unused connections are lost while new connections are established. Nerve cells can migrate within the brain, cell extensions can change direction and make new budding. As the brain matures, nerve cells grow, develop extensions to connect with other cells, and then form multiple connections at specific sites. While each neuron in a newborn baby’s brain makes an average of 2,500 synapses, this number increases to 15,000 during the developmental period. In adulthood, a decrease is observed in the number of synapses that are not used. The brain’s ability to form new synapses in adulthood is a topic that has been heavily researched.
The formation of new neurons in the adult human brain is very limited. New neurons are formed in the dentate fold of the hippocampus, a region responsible for memory and emotions, and in the sub-ventricular area of the lateral ventricle. Although the formation of neurons in these regions is not considered as neuroplasticity, it may contribute to the recovery of brain injuries. Plasticity in the adult brain occurs not by an increase in the number of cells, but by the regulation of intercellular connections.
Even in old age, new knowledge, skills and even new languages can be learned. This fact shows that the learning and remembering capacity of the brain continues in old age. In other words, structural and biochemical changes in nerve cell connections occur at all ages. Doing a movement over and over again allows the brain to learn this movement. This fact about the healthy brain is also true for the brain damaged by stroke or trauma. With plenty of repetition, the damaged brain can relearn lost skills. Unfortunately, not every stroke or brain injury patient can be 100% cured. Sometimes, when plasticity is left to its natural process, the direction it moves may not provide the desired functionality. The degree of improvement is age (young people have a better chance of recovery), It depends on the size of the damaged area and, most importantly, on the rehabilitation treatments applied. With rehabilitation, plasticity is tried to be improved in the right direction. Controlling plasticity is a subject of scientific studies in the field of neurological rehabilitation.
Neuroplasticity (plasticity in the brain) can be understood in several layers. The first layer deals with a single nerve cell and the events in that cell. The second layer covers the behavior of groups of neurons specialized for a particular function. While some of the changes at the cellular level are rapid but temporary, some are slow, but the effect is long-lasting. One of the most rapid changes occurs in the form of an increase in the activity of some neural circuits (excitation) and a decrease in others (inhibition) by managing neuronal traffic. Many neuronal circuits in our central nervous system are normally under pressure, that is, inhibited. With the damage of the higher executive centers, the pressure can be removed and many activities that are not normally seen can be observed. Abnormal reflex after brain injury or stroke,
Another relatively rapid change is in the strength of certain synapses. Sequential excitation at a synapse or simultaneous excitation of multiple synapses acting on a neuron strengthens the relevant link, that is, leads to easier excitation in subsequent signals. This is expressed in terms of long-term potentiation (LTP), which means strengthening at the synapse, and long-term depression (LTD), which corresponds to weakening of the synapse. Even a few minutes of synaptic activity can lead to changes that can last for hours or even be permanent. These are changes that take place at the molecular level.
Another form of cellular changes is anatomical, that is, it involves the structural deformation of extensions of nerve cells called dendrites, budding and elongation. It can be said that the plastic process that takes place is more permanent when there are structural changes such as an increase in the number of synapses in certain neural pathways.
Research on Plasticity in the Brain
Much experimental research is ongoing to understand and influence plasticity. For example, frequencies of repetitive transcranial magnetic stimulation (rTMS) higher than 5 Hz increase the excitability of the brain, while frequencies below 1 Hz cause inhibition. In transcranial direct current stimulation, the increase and decrease in excitability depend on the placement of the cathode and anode (negative and positively charged electrode). The combination of brain stimulation and peripheral nerve stimulation may have different effects on plasticity.
As a result of scientific research, some principles have been found that have an important effect on the rehabilitation process of stroke and trauma-related brain damage. As knowledge about plasticity increases, new physical therapy methods are being developed to strengthen and manage it.
Use it or Lose it
Parts of our body compete with each other for representation in the brain. Overuse of a body part increases its representation in the brain. In the case of not using it, the representation in the brain shrinks. An example of this has been demonstrated in experiments with people who are blind and can read Braille by touching their fingers. The representation of the fingers they use to read in these people’s brains is significantly larger than that of other people. On the other hand, it was observed that the representation of the ankle in the brain decreased after a few weeks in people who remained immobile because their feet were placed in a cast. Apart from the brain damage caused by the main disease, stroke patients also experience regression in functions due to non-use and inactivity.
Primary Motor Cortex ve Premotor Cortex
Movement orders in the brain are formed in a part called the primary motor cortex. This is the main area responsible for the movement. However, the part called the premotor cortex also contributes. Normally, the signal outputs of the premotor cortex are directed to the primary motor cortex. To a lesser extent, it can also generate signals that go to the muscles through the spinal cord, called the corticospinal tract. If the primary motor cortex is damaged, the premotor cortex can be the source of movement orders. This is one of the mechanisms by which stroke patients recover from stroke.
Compensation of the Opposite Hemisphere
In humans, one hemisphere of the brain controls the opposite half of the body. However, there are also neural pathways that run from one hemisphere of the brain to the muscles on the same side of the body. These pathways may play a role in the recovery of stroke patients. So the healthy brain side may be controlling both sides of the body. Another important point is the connections between the two hemispheres. Some nerve pathways from one brain hemisphere to the other can have a suppressive (inhibitory) effect. If these inhibitory connections are blocked, the excitability of the damaged brain may increase. Studies have shown that when bilateral movements are performed, blood flow to the damaged brain side increases. This finding is consistent with the idea that the activity of the intact hemisphere supports the damaged side.
Rehabilitation acts by reinforcing plasticity.
It has been observed that neuroplasticity mechanisms can be strengthened and facilitated in some cases. For example, the brain is more “plastic” in the first 3 months after a brain injury. PhysiotheraphyThe sooner and more intensively it is started, the better the return of movement function. This has been seen in studies of compulsive use therapy. Even in patients who are thought to have become chronic and unchanging, improvements can be achieved with intensive rehabilitation. Aerobic exercises increase the success of rehabilitation by positively affecting general health and well-being; In addition, it is thought to activate molecular mechanisms that facilitate plasticity. Applications such as neuromuscular electrical stimulation, robot-assisted physical therapy and virtual reality therapy are also effective with the principle of increasing plasticity. Long-term sensory stimuli expand the representation of the relevant muscles in the brain. Sensory stimulation can be done in many different ways, from passive movements to various electrical currents and acupuncture. Sensory stimuli are more effective if they are made interesting. The rich social interaction of stroke and brain injury patients also has a positive effect on plasticity.
New Techniques to Increase Plasticity in the Brain
Combination of drug therapy and physical therapy can increase the success of stroke and brain injury recovery. Although the effect of various drugs on plasticity has been investigated, there is no routine use of drugs to increase the effect of physical therapy yet. Further studies are underway to surpass the natural plasticity of the brain. Stem cell studies are the most popular example of this. While the production of new neurons in the normal adult brain is considered to be practically absent, this is attempted to be achieved with stem cell therapies. Another approach is neuroprostheses and brain-computer interfaces. It is thought that with these methods, the undamaged areas of the brain can be trained to take over the lost functions.